Fixed timer counting direction on all platforms

On some platforms, the hardware timers were counting down instead
of up, which broke the assumptions of the new timer implementation.
Fixed by inverting the timer value (relative to its maximum value).

src/common_tmr.c

@@ -272,6 +272,10 @@ timer_data_type platform_timer_op( unsigned id, int op, timer_data_type data )
       return platform_timer_get_diff_us( id, 0, platform_timer_get_max_cnt( id ) );
     else if( op == PLATFORM_TIMER_OP_GET_MIN_DELAY )
       return platform_timer_get_diff_us( id, 0, 1 );
+#ifdef PLATFORM_TMR_COUNTS_DOWN
+    else if( op == PLATFORM_TIMER_OP_START || op == PLATFORM_TIMER_OP_READ )
+      return platform_s_timer_op( id, PLATFORM_TIMER_OP_GET_MAX_CNT, 0 ) - platform_s_timer_op( id, op, 0 );
+#endif
     else
       return platform_s_timer_op( id, op, data );
   }

src/platform/lm3s/platform_conf.h

@@ -41,6 +41,7 @@
 #define BUILD_C_INT_HANDLERS
 
 #define PLATFORM_HAS_SYSTIMER
+#define PLATFORM_TMR_COUNTS_DOWN
 
 // *****************************************************************************
 // UART/Timer IDs configuration data (used in main.c)

src/platform/lpc288x/platform_conf.h

@@ -26,6 +26,8 @@
 #define TERM_LINES            25
 #define TERM_COLS             80
 
+#define PLATFORM_TMR_COUNTS_DOWN
+
 // *****************************************************************************
 // Configuration data